The same reason we don't manually control our digestion system: We have no need to.
Deeper explanation: Predators have their eyes both facing forward so they can judge distance (depth perception) when hunting prey. If we want to look left, we can look left. If we want to look right, we can do that too. It doesn't take long for you to look left and right and have an idea of your surroundings. Whilst true that it takes more energy to keep looking left and right, the quality of our vision is better because you have this contant depth perception.
Animals which are hunted from all directions (like fish) have eyes on the sides of their head so they can have a greater arc of vision to see oncoming predators. This requires less energy, but they have no depth perception. Luckily, they don't need it. They don't need to know how close a predator is, they need to know WHERE it is, and swim as fast as possible in the opposite direction.
We're predators. We need to see what's infront of us so we can hunt and navigate more easily.
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